Thermal treatment of aluminum alloys containing copper



Patented Dec. 1, 1936 UNITED STATES THERMAL TREATMENT OF AL ALLOYS CONTAINING COPPER Joseph A. Nook, Jr., Tarentum, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a. corporation of Pennsylvania No Drawing. Application April 21, 1932,

Serial No. 606,756

16 Claims. (Cl. 148-211) This invention relates generally to the heattreatable strong aluminum-base alloys containing copper, with or without one or more other elements such as zinc, nickel, and silicon, and with or 5 without the so-called hardeners or hardening elements manganese, chromium, zirconium, molybdenum, beryllium, boron, and titanium, one of the important objects of the invention is to provide thermally treated articles of such alloys, possesslO ing greater improvement in one or another physical property than has heretofore been obtainable by thermal treatment. Another object of the invention is to provide articles composed of alloys of the type indicated, with improvement particularly in the direction of yield strength in the artificially aged condition. Another object is the provision of high hardness in articles composed of such alloys. A further object is to provide alloy articles which will have improved resistance to 20 corrosion in the artificially aged condition. These and other objects I attain with these alloys by the addition thereto of a small amount of tin,

not exceeding 0.1 per cent and preferably not more than about 0.05 per cent. In fact, I have 5 found beneficial resultst be obtained with as little as 0.005 per cent. The advantages of the invention are, however, obtained only when the alloy is free from magnesium, either entirely or when the metal is present only in amount so 30 small as to be a mere impurity, not exceeding, say, about 0.1 per cent. The alloys to which my invention is directed are those in which the copper content is between about 2 and 12'per cent, with or without a total of 0.1 to 3 per cent of one or more of the so-called hardening elements mentioned above, and with or without one or more of the elements zinc, nickel, and silicon, the rest of the alloy being aluminum.

In amounts of about 0.15 to 15 per cent, tin

40 has been known as an alloying element which in aluminum-base alloys containing copper increases the fluidity and improves the machining and polishing characteristics of the alloy. Its use, however, has been generally discontinued, it hav- 5 ing been learned, as investigators have pointed out, that tin in the amounts heretofore used adversely affects the hot working characteristics of aluminum and-aluminum-base alloys, diminishes the corrosion-resistance of such materials, and, 50 generally, serves no useful function not more advantageously obtained with other alloying elemerits. V

Contrary to the accepted opinion and trend of the art, I have discovered that in alloys of certain 5 composition certain small amounts of tin are beneficial and desirable. In accordance with these discoveries and as a result of a series of experiments directed thereto, I have determined that tin is extremely beneficial and desirable in aluminum alloys when (1) the tin is present in 5 amounts of 0.005 to 0.1 per cent by weight; (2) the alloy contains cop'per; (3) the alloy contains no magnesium or contains that metal only in so small an amount as to constitute a mere impurity;

(4) the alloy is artificially aged, that is to say, when the alloy is subjected to artificial aging (preferably but not necessarily'after high temperature heat-treatment), say at a temperature between about 100 and 200 C.

The benefits of my invention appear to be due to the response of a peculiar internal alloy-structure to the artificial aging treatment. Aging phenomena in aluminum-base alloys are believed to be the result of the precipitation of an alloying element from a solid solution thereof in aluminum which is super-saturated with respect thereto. The precipitation is submicroscopicor on the border line between submicroscopic and microscopic. By careful methods, however, it is possible to prepare metal specimens which, under the action of an etching agent, reveal a structure A indicative of the artificially aged condition of the metal. r

For example, a section of an artificially aged wrought aluminum alloy article composed of 4 0 per cent of copper without tin, etched with a mixture of hydrofluoric, hydrochloric and nitric acids, shows under a magnification of 500 diameters an aluminum matrix composed of contrasting grains having distinctly marked boundaries. Particles .of the constituent CuAlz are seen scattered through the matrix but substantially none are found in the grain boundaries. The same alloy containing 0.05 per cent of tin shows after the same artificial aging only slight grain ,con- 0 trast, the grain boundaries are distinctly less sharp, and they contain multitudes of small particles of CuAlz. According to the theories of submicroscopic precipitation, the differences in structure noted in the tin-containing alloy indicate a more advanced stage of submicroscopic precipitation of C11A12, in fact showing that the precipitation has advanced to a large extent beyond the submicroscopic to the microscopic stage. This is evidenced by the particles of CuAlz in the grain boundaries, resulting from coalescence of particles precipitated in submicroscopic size} The enhanced aging phenomenon indicated by the above described aluminum alloys containingtin is strongly evidenced by the development in loy casting containing 11.78 per cent of copper.

and 0.05 per cent of tin, heat-treated for 16 hours at 515 C. and aged for 15 hours at 150 C., had a Brinell hardness of 122. The same alloy without tin, similarly heat-treated and aged, had a Brinell hardness of only 107. Similarly, and under the same treatment, an alloy containing about 4 per cent of copper, about 10 per cent of silicon,

. and about 0.04 percent of tin, developed a Brinell hardness of 124, while a similar alloy not containing tin developed a Brinell hardness of only A further effect of the tin addition upon aging is particularly evidenced in certain specially valuable and preferred alloys.. Under the influence of aging treatments, aluminum alloys containing 2 to 6.5 per cent of copper, 0.005 to 0.1 per cent of tin, and substantially free from magnesium, developed yield strengths which are on the order of 30 to 200 per cent greater than the yield strengths of similar alloys not containing tin. While the fundamental reasons for such increase in yield point are obscure, the effect is very pronounced. Forinstance, a magnesiumfree aluminum alloy containing 4 per cent of copper and 0.05 per cent of tin was heat-treated at 510 C. for 20 minutes, quenched to room tem-. perature, and artificially aged for 18 hours at 150 C. This alloy had a yield point of 43,000 pounds per square inch. A similar alloy, similarly treated but not containing tin, had a yieldpoint of only 20,200 pounds per square inch.

The aluminum base alloys which are improved by the enhanced aging induced therein by the addition of small amounts of tin are those containing 2 to 12 per cent of copper, with or without certain other alloying elements which I have found to be useful in modifying the general properties of the alloy without masking or destroying the beneficial properties above noted. Thus the aluminum-copper alloys may contain 0.1 to 3 per cent of a class of hardening elements which may be present, separately or together; each, however, not exceeding greatly the following limits:

manganese 0.1 to 2 per cent, chromium 0.1 to 1 per cent, boron 0.1 to 0.5 per cent, molybdenum 0.1 to 1 per cent, zirconium 0.1 to 0.5 per cent, beryllium 0.1 to 2 per cent, and titanium 0.03 to 0.5 per cent.

The alloys may also contain 0.1 to 14 per cent of alloying elements of the class defined to be zinc, nickel and silicon. The nickel should not be present in amounts of more than 7 per cent.

The zinc and silicon may each be present in amount as high as 14 per cent but the total should not exceed that figure.

I have determined that "magnesium is a harmful addition to ,the alloys above described in that its presence in substantial amounts destroys in large part the effects induced by the addition of small amounts of tin.

The preferred alloys are those in which the enhanced aging is evidenced by a marked increase in yield point. These alloys, as above noted, contain'2 to 6.5 per cent of copper and 0.005 to 0.1 per cent of tin. They are characterized in the artificially aged condition by a yield point substantially higher than that of the same alloy devoid of tin. In their preferred form, these ailoys may also contain 0.1 to 1 per cent, in total,

of one or more of the hardening elements above mentioned, with or without 0.1 to per cent, in total, of an element or elements of the class herein defined to consist of zinc, nickel and silicon. As specifically illustrating these alloys, I have selected a wrought aluminum-base alloy without magnesium, containing about 4.4 per cent of copper, about 0.85 per cent of manganese, about 0.75 per cent of silicon, about 0.4 per cent of iron. Alloys of this composition, with and without the addition of 0.05 per cent of tin, were heat-treated at 520 C. for 15 minutes, quenched in water, and subjected for 18 hours to an aging treatment at 143 C. The tin-free alloy had a tensile strength of 58,700 pounds per square inch, a yield strength of 33,250 pounds per square inch, and an elongation of 16.8 per cent in two inches. The alloy containing 0.05 per cent of tin had a tensile strength of 63,190 pounds per square inch, a yield strength of 46,250 pounds per square inch, and an elongation of 11 per cent in two inches.

The aging treatments and heat-treatments to which the above-mentioned alloys are subjected in order to develop their advantageous properties are the thermal treatments well known to the art. The heat-treatment usually comprises heating the aluminum-copper alloys to over about 400 C. but below the temperature at which the lowest melting constituent of the alloy becomes molten, generally known as the point of incipient fusion. The alloy thus treated is, in the preferred practice, cooled rapidly, as by quenching in water or air, to room temperature. The artificial aging usually comprises heating the aluminum-copper alloy to temperatures of about 100 to 200 C. until the desired increase in properties is obtained. The artificial aging in the preferred practice of the invention is preceded by heat-treatment, but the enhanced aging I resistance of the alloy to corrosion, but I have found that when these alloys, especially those of the preferred copper content (2 to 6.5 per cent as stated above), contain tin in the amount prescribed by my invention this detrimental result is considerably lessened by the enhanced aging efiect. In particular the artificially aged alloys show'a marked decrease in propensity-to undergo intercrystalline or intergranular corrosion, a type of corrosion which is more objectionable than the ordinary surface type because it is often not readily apparent and so is apt to escape observation until the corroded part or article fails as a result of the internal weakening.

The aluminum base alloys herein described and claimed are those containing at least 70 per cent of aluminum, which metal may contain impurities, such as amounts of iron up to about 1.5 per cent and, likewise, small amounts of silicon such as are known to occur in virgin aluminum.

The enhanced aging herein described as resulting from the addition of tin in the stated amount to magnesium-freealuminum alloys containing castings of such alloys containing tin, I have found that heat treatment at elevated temper- Gil atures without artificial aging produces a higher ductility than is obtainable by heat-treatment of a casting of the same alloy without the tin. This species of the invention I do not claim specifically herein but do so in my copending application Serial No. 606,755, filed of even date herewith, and issued as United States Letters Patent 2,022,- 686 under date of December 3, 1935.

In the appended claims the term tensile property, or the like, is intended to include hardness as a property which can be favorably afiected by the enhanced artificial aging produced by my invention. Also within the spirit of the appended claims the article may be an ingot or other body designed for further casting or for working or it may be a cast or wrought article which is suitable for immediate use or sale or which may require some further operation to fit it for use .or sale.

I claim 1. An article of artificially aged alloy free from magnesium and composed of aluminum; 2 to 12 v per cent of copper; at least one hardening element of the class consisting of manganese 0.1 to 2 per cent, chromium 0.1 to 1 per cent, boron 0.1 to 0.5 per cent, molybdenum 0.1 to 1 per cent, zirconium 0.1 to 0.5 per cent, beryllium 0.1 to 2 per cent, and titanium 0.03 to 0.5 per cent, the total amount of hardening elements being 0.1 to 3 per cent; and 0.005 to 0.1 per cent of tin.

2. An article of thermally treated aluminumcopper alloy free from magnesium, containing'2 to 12 per cent copper and 0,005 to 0.1 per cent of tin, the remainder being essentially aluminum; the alloy having the structure charactercopper 2 to 6.5 per cent, and tin 0.05 to 0.1 per cent, the remainder of the alloy being essentially aluminum.

5. An article of artificially aged aluminumcopper alloy free from magnesium, containing 2 to 12 per cent of copper; 0.005 to 0.1 per cent of tin; and at least one element of 'the class of hardeners composed of manganese, chromium, boron, molybdenum, zirconium, beryllium and titanium, in total amount from 0.1 to 3 per cent;

the remainder of the alloy being essentially alu minum.

6. An article of artificially aged aluminume copper alloy containing copper 2 to 6.5 per cent, tin 0.05 to 0.1 per cent; and at least one element of the class of hardeners composed of beryllium, boron, manganese, molybdenum, chromium, titanium and zirconium, the total hardening addition being from 0.1 to 3 per cent; the remainder of the alloy being essentially aluminum.

7. An article of artificially aged aluminumcopper alloy containing copper 2 to 6.5 per cent, tin 0.05 to 0.1 per cent; and at least one element of the class of hardeners composed of beryllium, boron, manganese, molybdenum, chromium, titanium and zirconium, the total hardening addition being from 0.1 to 1 per cent; the remainder of .the alloy being essentially aluminum.

8. Ina method of making an article of aluminum-copper alloy, the steps comprising forming an article of a magnesium-free. aluminum-copper alloy containing copper 2 to 12 per cent and tin 0.005 to 0.1 per cent, the remainder essentially aluminum, and artificially aging the article whereby a tensile property of the alloy is improved over that of a like alloy free from tin.

9. In a method of making an article of aluminum-copper alloy, the steps comprising forming an article of a magnesium-free aluminum-copper alloy containing copper 2 to 6.5 per cent and tin 0.05 to 0.1 per cent, the remainder being essentialiy aluminum, and artificially aging the article whereby a tensile property of the alloy is improved over that of a like alloy free from tin.

10'. In a. method of making an article of aluminum-copper alloy, the steps comprising forming an article of a magnesium-free aluminum-copper alloy containing copper 2 to 12 per cent; tin 0.005 to 0.1 per cent; and at least one hardening element of the class consisting of manganese 0.1 to 2 per cent, chromium 0.1 to 1 per cent, boron 0.1 to 0.5 per cent, molybdenum 0.1 to 1 per cent, zirconium 0.1 to 0.5 per cent, beryllium 0.1 to 2 per cent, and titanium 0.03 to 0.5 per cent, the total hardening content being 0.1 to 3 per cent, and the remainder being essentially aluminum; and artificially aging the article whereby a tensile property of the alloy is improved over that of a like alloy free from tin.

num-copper alloy, the steps comprising forming an article of a magnesium-free aluminum-copper alloy containing copper 2 to 12 per cent; tin

0.005 to 0.1 per cent; and at least one hardening element of the class consisting of manganese 0.1 to 2 per cent, chromium 0.1 to 1 per cent, boron 0.1 to 0.5 per cent, molybdenum 0.1 to 1 per cent, zirconium 0.1 to 0.5 per cent, beryllium 0.1 to 2 per cent, and titanium 0.03 to 0.5 per cent, the total hardening content being 0.1 to 3 per cent, and the remainder being essentially aluminum; and artificially aging the article between about C. and 200 C. inclusive, whereby a tensile property of the alloy is improved over that of a like alloy free from tin.

13. In a method of making an article of aluminum-copper alloy, the steps comprising forming an article of a magnesium-free aluminum-copper alloy containing copper 2 to 12 per cent and tin 0.005 to 0.1 per cent, the remainder being essentially aluminum; heat treating the article between about 400 C, and the temperature of incipient fusion; and artificially aging the article between about 100 C. and 200 C. inclusive, 75

whereby a tensile property of the alloy is improved over that of a like alloy free from tin.

14. In a method of making an article of aluminum-copper alloy, the steps comprising forming an article of a magnesium-free aluminum-copper alloy containing copper 2 to 6.5 per cent and tin 0.05 to 0.1 per cent, the remainder being essentially aluminum; heat treating the article between about 400" C. and the temperature of incipient fusion; and artificially aging the article between about 100 C. and 200 C. inclusive, whereby a tensile property of the alloy is improved over that of a like alloy free from tin. I

15. In a method of making an article of aluminum-copper alloy, the steps comprising forming an article of a magnesium-free aluminum-copper alloy containing copper 2 to 12 per cent; tin 0.005 to 0.1 per cent; and at least one hardening element of the class consisting of manganese 0.1

2 to 2 per cent, chromium 0.1 to 1 per cent, boron 0.1 to 0.5 per cent, molybdenum 0.1 to 1 per cent,

' zirconium 0.1 to 0.5 per cent, beryllium 0.1 to 2 and the remainder being essentially aluminum;

\ heat treating the article between about 400 C.

and the temperature of incipient fusion; and

artificially aging the article between about 100 C. and 200 C. inclusive, whereby a tensile property of the alloy is improved over that of a like alloy free from tin.

16; In a method'of making an article of aluminum-copper alloy, the steps comprising'forming an article of a magnesium-free aluminum-copper alloy containing copper 2 to 6.5 per cent; tin 0.05 to 0,1 per cent; and at least one hardening element of the class consisting of manganese, chromium, boron, molybdenum, zirconium, beryllium and titanium, the total hardening content being 0.1 to 1 per cent, and the remainder being essentially aluminum; heat treating the article between about 400 C. and the temperature of incipient fusion; and artificially aging the article between about 100 C. and 200 C. inclusive,

whereby a tensile property of the alloy is improved over that of a like alloy free from tin.

JOSEPH A. NOCK, JR. 

